Rogue rootworms: Managing resistance

Rootworm-resistant traits remain a good way to protect corn hybrids from corn rootworm. Still, cracks in the trait armor continue to appear.

In Iowa, Iowa State University (ISU) entomologists have confirmed rootworm resistance to Cry3Bb1 and mCry3A Bt proteins. The Monsanto trait in the YieldGard RW group contains the Cry3Bb1 protein. Syngenta’s Agrisure RW trait in several Agrisure trait products contains the mCry3A protein. So far, entomologists have just confirmed resistance in the western corn rootworm Northern corn rootworm does not yet resist Bt traits.

ISU researchers also confirmed cross-resistance between the two traits. “If you have rootworms that resist the Cry3Bb1 protein and switch to the mCry3a protein, you can still expect root injury,” says Erin Hodgson, ISU Extension entomologist.

In 2012, University of Illinois entomologists confirmed rootworm resistance to the Cry3Bb1 protein in Illinois. Resistance has not been confirmed in the Cry34/35Ab1 protein that is contained in Dow and DuPont Pioneer Herculex RW trait packages.

Even where resistance has not been confirmed, though, performance problems have surfaced to the Cry3Bb1 and mCry3A proteins.

“The problem is growing in areas like southern Minnesota, northern Iowa, eastern South Dakota, and eastern Nebraska,” says Hodgson. “That is not only in range and number of fields with problems with poor performance, but also in different kinds of hybrids. The problem is primarily in areas of continuous corn. It should be high on your radar of things to look for.”

Cases where resistance is not confirmed but performance is subpar is termed unexpected damage.

“In a lot of cases where you see unexpected damage, fields have been corn-on-corn for many years,” says Miloud Araba, Syngenta technical product lead for corn traits. “You can be in corn for just two years and have a high population in that particular field. You can also have high damage in just part of a field.”

High rootworm populations can pressure all traits, regardless of whether resistance has been confirmed, says Ken Ostlie, University of Minnesota Extension entomologist.

He notes a Springfield, Minnesota, field that was overwhelmed by corn rootworm in 2012. Rootworms resisted the Cry3Bb1 protein contained in the Monsanto Genuity VT Triple Pro hybrid that the farmer had planted for several years.

Cross-resistance to the Agrisure RW hybrid also occurred in the field. Similar results happened in 2013. However, performance of a pyramided SmartStax (two rootworm traits) and Herculex products containing the Cry34/35Ab1 protein also had excessive root damage.

“As resistance developed to the Cry3Bb protein, rootworm numbers built up,” explains Ostlie. “There were huge numbers of beetles in the field. In high-pressure situations, all traits may be at risk. You don’t get high population levels like these unless you have resistance to one or more traits.”